1. Field of the Invention
This invention relates to fabrication of x-ray masks and, in particular, masks including a patterned metal on a membrane.
2. Art Background
As design rules in the manufacture of devices, e.g. integrated circuits opto-electronic devices, and micro-mechanical structures, become smaller, the radiation employed for lithography, in turn, must be of a correspondingly shorter wavelength. Thus, for example, when the design rule is below 0.5.mu., use of short wavelength radiation such as x-ray radiation (radiation having a wavelength typically in the range 4 to 150 .ANG.) has been suggested.
During exposure, energy incident on a mask which defines a pattern is transmitted in this pattern to expose an underlying energy sensitive material. The energy sensitive material after this exposure is delineated into the pattern by development and employed in the manufacture of the desired device. For x-ray exposure, the mask is generally a membrane stretched across a supporting structure, for example, a ring with a region patterned in a metal coating the membrane surface. Typically, the membrane is a material such as Si, SiN.sub.x (x is typically between 1 and 1.3) or SiC, and has a thickness generally in the range 0.1 to 4 .mu.m.
Since the membranes must be quite thin to avoid excessive attenuation of incident energy, substantial stress, i.e. stress greater than 50 MPa, imposed on the membrane from the overlying metal pattern is unacceptable because it causes unacceptable distortion of the pattern. The requirement of limited stress, in turn, imposes substantial limitations on the process of forming the overlying metal pattern.
In a typical mask fabrication procedure, a layer of metal is deposited on a membrane such as by sputtering. A pattern in polymeric material is formed over the metal layer, and the metal regions not covered by the polymeric material are removed by etching. Subsequent removal of the overlying polymeric material leaves a patterned metal overlying the membrane.
Various materials have been suggested for use in the metal layer. Although gold is relatively easy to deposit, its presence in device manufacturing environments and in particular, integrated circuit manufacturing environments, is not preferred. Gold impurities, even in extremely small mounts, introduced into an integrated circuit often substantially degrade the properties and reliability of the device. Stress in gold films is also known to change with time, even at room temperature. Recent studies indicate that at temperatures above 70.degree. C., stresses increase rapidly. Therefore, materials other than gold have been investigated.
One alternative to gold is tungsten. Although tungsten is considered compatible with an integrated circuit manufacturing environment, tungsten films deposited on a membrane generally induce substantial compressive or tensile stress that ultimately distorts the required pattern or even produces membrane failure. Various attempts have been made to reduce the stress associated with the deposition of tungsten. For example, as described by Y. C. Ku et al, Journal of Vacuum Science & Technology, B9, 3297 (1991), a monitoring method is employed for determining stress in the tungsten being deposited. This monitoring method is based on the resonant frequency f of a circular diaphragm of the composite structure which, in turn, is related to the stress by the equation: ##EQU1## where r is the radius of the membrane, .sigma..sub.m, .rho..sub.m, and t.sub.m are stress, density, and thickness of the membrane respectively, and the corresponding terms such as .sigma..sub.f are stress, density, and thickness respectively, of the film. Since the density of the film and membrane are generally known, this equation allows calculation of stress once the resonant frequency and film thickness are measured.
Ku and coworkers, used a commercially available optical distance measuring device to monitor diaphragm position. Movement of the diaphragm was induced by electrostatic forces applied to the diaphragm from an electronic oscillator-driven capacitively coupled electrode. The oscillator frequency was slowly swept to allow location of the diaphragm mechanical resonance and from this value, the stress was determined.